A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be applied to a next-generation flat panel display. In particular, a light-emitting device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
The light emission mechanism of a light-emitting element is said to be as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from the cathode and holes injected from the anode recombine in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons return to the ground state. Singlet excitation and triplet excitation are known as excited states, and it is thought that light emission can be achieved through either of the excited states.
In order to improve the characteristics of a light-emitting device including such light-emitting elements, improvement of an element structure, development of materials, and the like have been actively carried out (for example, see Patent Document 1).